Prescription eyeglass lenses are curved in such a way that light is correctly focused onto the retina of a patient's eye, improving vision. Such lenses are formed from glass or plastic lens “blanks” having certain desired properties to provide the correct prescription for the patient. The blanks are usually circular and of substantially larger dimension compared to the relatively smaller finished lenses assembled into eyeglass frames. Therefore, a lens blank must be edged to fit an eyeglass frame selected by the patient.
Ophthalmic laboratory technicians cut, grind, edge, and polish blanks according to prescriptions provided by dispensing opticians, optometrists, or ophthalmologists. The specifications include the patient's full prescription, including: 1) the total power the finished lens must have; 2) the strength and size of any segments, if needed (i.e. multifocal lenses); 3) the power and orientation of any cylinder curves; and 4) the location of the optical center and any inducted prism that may be needed.
In addition, the large diameter blank is sized and shaped to fit into the frame selected by the patient. The lens blank may be shaped using an edger, such as the edger disclosed in U.S. Pat. No. 6,203,409 to Kennedy et al., the disclosure of which is incorporated herein by reference. The blank is edged so that the periphery of the finished lenses fit into the openings on the frames.
Edging of a lens blank typically requires the application of a block to a surface thereof. The block is releaseably secured to a clamp assembly, so that rotation of the clamp assembly causes corresponding rotation of the lens blank. As the blank is rotated, the periphery of the blank may be cut to a desired size using a router tool. The blank may be either ground or cut. Wet edgers use diamond-impregnated wheels with different abrasive grits to grind the lens material. A coolant is sprayed on the wheels during edging to reduce heat. Dry edgers use carbide steel or diamond blades mounted on the spindle of a motor to shave the lens. The lens periphery may also be polished using a polishing tool. Some edgers are also able to form a bevel about the periphery of the lens.
Information relating to the size and shape of the lens needed for a particular frame (i.e. trace data) may be generated, and subsequently transmitted to the edger. Such trace data may be provided by frame manufacturers, or generated by a tracer machine. Trace data may be downloaded and/or transmitted to a storage medium in a control system, such as a central processing unit, in communication with the edger. The edger processes the edge of the lens blank to create an edge profile according to the trace data. The finished lens may then be assembled with the selected eyeglass frames.
In order to improve efficiency, some edgers use CNC (Computer Numeric Control) technology whereby a computer controls the lens processing equipment by following encoded commands. The commands are based on information from frame tracings or internal lens probes and the user. Information relating to the size and shape of the lens needed for a particular frame (i.e. trace data) may be generated, and subsequently transmitted to the edger. The trace data may be stored in the storage medium and recalled by the control system as needed.
Some lenses require that the lens contain drill features in the surface of the lens. For example, some frame assemblies require that one or more holes be drilled in the lenses, particularly lenses to be used in rimless style frames. Several factors to consider when determining the hole position include the horizontal and vertical coordinates, lens base curve, wrap angle, and the mounting's pantoscopic tilt. Hand drilling is used by some laboratories. Other laboratories use a drill press.
Typically, one drill bit is used to cut holes of varying sizes. In order to provide proper drill hole size, many conventional techniques require a technician to drill holes into a lens blank, and then make an estimation of the hole size correction needed. This is often a tedious and time consuming operation. In addition, accurate drill depth is required for optimal functioning of a lens drilling mechanism. Holes must typically be drilled completely through the lens blank. It is not always obvious to the technician that an adjustment is needed to achieve proper drill depth, particularly when drilling lens blanks having a relatively high wrap, such as frames having a curvature greater than 6 diopters.